In a typical modern office waste paper is separated from other wastes and collected for recycling. Nevertheless, paper suitable for recycling is rarely completely separated from other wastes which cannot be recycled as paper. Paper for recycling is often contaminated with plastic bags, transparent overhead view-foils, X-Ray film, envelope windows, and paper with high wet strain such as Express Mail envelopes and bible papers. Heavy contaminants such as metal and rock may also be present.
Because the cost of sorting recycled paper by hand is generally prohibitive, all the trash which is collected as recyclable paper is commonly placed in a pulping device where it is mixed with water and chemicals and made into a pumpable slurry. The slurry thus produced is about 12 to 18 percent pulp by weight. The slurry is diluted with water to about 4 percent fiber dry weight as it is allowed to flow to a de-trashing unit. A typical de-trashing unit consists of a cylindrical tank with one end forming a screen through which the slurry is drawn by a pump. To aid the passage of the slurry through the screen a three bladed impeller is mounted adjacent to the screen and caused to rotate.
Unfortunately, with existing de-trashing units the impeller blades shred the non-pulping contaminants such as plastic bags, view-foils, envelope windows, etc. The shredded material then passes through the screen in the de-trasher and must be removed by additional processing steps. Inevitably the pumps between the additional pieces of processing equipment shred the contaminants into ever smaller particles so that a certain portion of the contaminants end up in the finished paper. Removal is possible but involves considerable additional cost. Contaminants which are not removed result in a final product which is of lower quality and value.
In addition to shredding the contaminants, the rotor in existing de-trashing units becomes laden with a buildup of contaminants such as plastic bags which wrap around the impeller. This buildup of contaminants increases the power required for operating the de-trashing unit and requires cleaning of the unit every eight hours.
What is needed is an improved de-trashing unit which can separate trash without significantly shredding lightweight plastic and without becoming clogged with trash.